fix(history): log when a session's CSV file can't be opened

Recording writes and index persistence were already logged on failure,
but reading a session's data (for charting, end-of-session summaries,
orphaned-session recovery, and legacy log import) silently returned an
empty result on any I/O error — indistinguishable in the UI from a
session that legitimately has no data yet. A moved, deleted, or
permission-denied CSV now logs a warning with the path, so "why does
this session show no chart data" is diagnosable from the debug log
(surfaced in the Status window) instead of a dead end.

Refs #175